Various methods are known for sensing gas concentration in a gas stream. For detecting hydrogen sulfide, tape analyzers are typically used. Hydrogen sulfide (H2S) tape analyzers generally operate by causing a gas stream to enter a reaction chamber through an aperture and permitting the hydrogen sulfide in the gas steam to react with lead acetate on a portion of lead acetate-impregnated paper tape. As the lead acetate reacts with the H2S in the gas stream, lead sulfide is formed on the tape. As the reaction proceeds the lead sulfide build up is visible as a brown stain on the tape. Reflectance of light from the lead sulfide is measured using a photo-detector to produce a voltage signal representing instantaneous reflectance of light from the lead sulfide. After the reaction has occurred, the tape is advanced to expose a new portion of the tape in the reaction chamber.
The rate of change of the voltage signal (dV/dt) is proportional to the amount of H2S in the gas and the amount of lead acetate on the tape. The amount of lead acetate on the tape is generally considered to be constant in each section of the tape.
An analog to digital converter is used to produce values representing instantaneous amplitude of the voltage signal and these values are used to calculate a rate of change of the voltage signal at a particular point for use in calculating a concentration value indicating concentration of hydrogen sulfide in the gas stream.
The chemical reaction of hydrogen sulfide with lead acetate is not a linear process. When the lead acetate on the tape is initially exposed to the H2S, it has no lead sulfide build up, but as the chemical reaction occurs, less and less lead acetate remains on the tape as more and more lead sulfide builds up on the tape. Initially the reaction is fast, and then it slows down. Eventually, the reaction appears to reverse because the lead sulfide build up actually becomes reflective.
Successive measurements taken by most tape analyzers vary both in the time domain and the voltage domain. A specific absolute voltage will be detected at different times for different measurements of the same gas. The voltage (or slope) measured a specific amount of time after a measurement begins will be different from measurement to measurement. Consequently H2S Tape Analyzers are typically calibrated to a single gas which has an H2S concentration that is typically about 70% of the defined full scale range of the analyzer.
Errors in measurements taken by H2S tape analyzers result from various sources including the lead-acetate impregnated tape, the constituents in the gas stream itself, the aperture size through which the gas enters the reaction chamber, temperature of the reaction chamber and sampling rate. For example, the tape used in tape analyzers has properties that can vary from measurement to measurement. The chemical properties of the paper can affect the reaction, the concentration of lead acetate on the tape may vary along the length of the tape and the temperature of the tape at the time of measurement can affect the accuracy of the measurement. Apart from temperature, it is not practical to pre-determine the properties of the tape. The paper is generally considered to be inert in the reaction, and when the tape is manufactured the lead acetate solution that the tape is dipped into is replenished, and constantly stirred so that its concentration remains generally constant along the length of the tape.
The gas stream containing the H2S gas being analyzed has many properties which can affect the accuracy of a measurement. Pressure variations can affect the flow rate, and therefore the amount of H2S passing on to the tape. Contaminants such as methyl mercaptan can cause side reactions that can affect the ability of hydrogen sulfide to react with the lead acetate.
The aperture size through which the gas stream enters the reaction chamber can also have an effect on the accuracy of a measurement. Aperture size affects the amount of gas that approaches the surface of tape and can affect the evenness with which the gas contacts the surface of the tape. This affects the repeatability as well as reaction time for any given measurement.
The temperature of the reaction chamber also affects measurements. Increasing the temperature of the reaction chamber has the effect of increasing the rate of the chemical reaction while decreasing the temperature decreases the rate of the chemical reaction. Thus a measurement taken at a higher temperature will indicate a higher gas concentration, while a measurement taken at a lower temperature will indicate a lower gas concentration.
Sampling rate can also affect the accuracy of an H2S analysis. When measuring high concentrations of H2S, a low sample rate may not provide sufficient granularity in measurements for an accurate concentration value to be calculated. When measuring low concentrations with a high sample rate an excessive number of samples may be taken.
Typically, lead acetate H2S tape analyzers specify measurement error based on Full Scale. For instance, a Galvanic Applied Sciences H2S Analyzer will specify a full scale error of 1.0%. In the case of a low concentration analyzer, the full scale may be 10 PPM and a typical gas test concentration would be about 1 PPM. The Full Scale error would be 1.0% while the error on reading would be 10%.
Most H2S tape analyzers on the market today use a time-based analysis procedure to produce a concentration value. This type of analysis typically achieves an accuracy of 1.0% Standard Deviation on Full Scale.
In a time-based analysis procedure, the rate of change of voltage (dV/dt) produced by the photodetector is averaged over a fixed time interval beginning at a pre-defined time. For example, each measurement could involve averaging dV/dt from the beginning of minute 3 to the end of minute 3. Measurements taken by this procedure are highly influenced by the effects described above and thus are neither sufficiently linear nor sufficiently repeatable for accurate, reliable measurements. Consequently, many H2S Tape Analyzer manufacturers advertise the error of measurement produced by their devices to be “Error on Full Scale” and limit the dynamic range of their analyzers.
A practical way to ensure linearity and repeatability in a time-based analysis procedure in an H2S analyzer is to reduce the dynamic range of the analyzer itself. This is done by selecting an aperture size suitable for the range of H2S concentrations being measured, calibrating with a gas close to the concentration of the gas being measured, and controlling the temperature of the reaction chamber, the incoming gas stream and/or the temperature of the tape.
Time based measurement techniques however, enable easy determination of maintenance intervals for replacement of tape cartridges.
An alternative way of measuring concentration involves a voltage-based analysis process. The voltage based analysis process is especially useful for taking measurements of gas streams having a relatively high concentration of H2S. Essentially, the voltage-based process involves initiating an acquisition process when a certain threshold voltage level is detected in the signal produced by the photo detector. The acquisition process results in the acquisition of a dV/dt value acquired at a time based on the threshold voltage. The length of the time required to execute the process is typically proportional to the amount of time it takes for the reaction to produce enough lead sulfide to reflect enough light to cause the photo-detector to reach the threshold voltage.
The voltage based analysis process is based on an assumption that that the peak dV/dt occurs at roughly the same absolute voltage in the range of concentrations expected to be measured. In addition, the voltage based analysis process relies on the assumption that the peak is relatively short for high concentrations, and relatively long for low concentrations.
When compared to the timed based analysis procedure, the voltage based analysis procedure is improved in almost all respects. However, in practice measurements are only marginally better because the maximum dV/dt does not always occur at the same voltage. Thus, the analysis is non-linear and this limits the dynamic range of the technique. In short, both the time based analysis procedure and the voltage based procedure have shortcomings that need to be overcome.